1. Field of the Invention
The present invention relates to sampling devices for obtaining representative samples from a bulk quantity of material.
2. Description of the Prior Art
Particulate materials used in a process often at some time or another must be withdrawn from a larger mass of material. The sample withdrawn may be used in the actual process or may be used for analysis as a control. Wherever it is assumed that the sample is the same, or representative of the bulk material, that sample must be obtained in a statistically valid manner.
It is difficult to obtain a representative sample of material from a bulk quantity of a mixture of particles in a fluid. If the mixture is freely flowing like dry sand in air, the large particles tend to separate from the small ones and locate at one or more boundaries within the mixture. In the case of cohesive materials, like cornstarch or viscous pastes, it is difficult to cause the material to flow so that sampling from the entire mixture is impossible.
Many techniques have been proposed and are being used for sampling powders and suspensions. In the simplest technique, a sample of powder or suspension is removed from the exposed surface of the bulk material, using a scoop or similar receiver. This method gives samples which have been taken from the outer areas of the material and are thus liable to be non-representative of the bulk. The technique is used extensively since it is rapid and inexpensive, but it is unsuitable for freely flowing powder since these roll off the scoop.
Many more complex sampling techniques have been devised, all of which have some disadvantages. An extensive study of sampling devices indicates that the spinning riffler is very efficient. This device consists of a ring of containers rotating under a powder feed so that each time a container passes under the stream of powder, it collects a small portion. The sample in each container will thus be made up of many small portions drawn from all points of the bulk. A temporary spinning riffler can be constructed simply with a phonograph turntable, a supply of paper cups in close proximity to the periphery of the turntable with a funnel or hopper positioned above the ring of paper cups. Such a device is disclosed in the article "Determining the Characteristics of Fine Powders", by Brian H. Kaye. pages 239 and 240 of Chemical Engineering for November 7, 1966.
In U.S. Pat. No. 797,144, a sampler or rotating riffler is disclosed which employs a horizontal rotating disk with sample containers supported thereby which is mounted on a base containing a drive. A volume of particles is fed into a funnel-like device wherby it drops in a controlled flow into the sampling containers rotating below the funnel.
In U.S. Pat. No. 2,441,774, a rotating riffler type device is disclosed which employs a rotatable assembly driven by a variable speed drive. The volume of particles is fed into a funnel-like device from which they drop in a controlled flow into sampling containers rotating below the funnel. The rotatable support for the sampling containers is a ring which is supported from above by tie rods. Antispill devices are provided between the sampling containers.
In U.S. Pat. No. 3,215,173, a bag-filling device is disclosed which rotates and has a series of funnels around the periphery of a round upper portion which are fed from a conveyor belt as the apparatus rotates. Below these funnels is a space for attaching bags which rest on weighing devices, and the entire apparatus is supported by peripheral bearings. However, this apparatus is not a riffler for samples but rather a device for filling bags which move one at a time under the conveyor belt rather than rapidly rotating to get a representative sample.
In U.S. Pat. No. 3,667,512, a sampling device is disclosed for obtaining representative samples of materials from a bulk quantity thereof which includes a plurality of rotating sample containers which are sequentially and repetitively fed by a flow stream of the material which emanates from a hopper. A screw conveyor mechanism controls the flow rate of the stream to insure representativeness and statistical validity of the samples.
The rotating riffler type devices described above are substantial improvement over the other prior art devices. However, a need exists for sampling means which can be used both in the case of extremely free-flowing materials or highly cohesive materials to get a sample which is statistically valid and identical in composition to the original batch. More specifically, the devices described above represent a problem if the containers are merely loosely placed upon a disk such as the use of paper cups on a phonograph turntable. With such devices, if the disk is rotated rapidly enough, which may be necessary with some materials, the paper cups would fly off the disk. Accordingly, some means is generally needed to retain the containers on the disk and the prior art devices for this purpose generally are adapted for only one size of container. If the riffler is to be used for a variety of materials, it is often necessary to employ a variety of containers and devices providing for variety of size and shape of containers.
Also, in general the disk or similar member is mounted on a single shaft. Generally due to flexibility of the disk and shaft and/or inaccurate mounting of the disk on the shaft which serves both the function of rotating and supporting this disk, the disk will not remain level but will flex due to a cantilever effect. Such flexing can result in uneven distribution of samples among the cups or the containers. In addition, with the prior art devices, considerable material is wasted and lost, particularly where the containers are round in cross-section, by spill-over as the cups pass beneath the discharge from the hopper or funnel.
Accordingly, it is the purpose of the instant invention to provide an apparatus for obtaining a representative sample of material from a larger batch or bulk of the material which is free from the above deficiencies.